1. Field of the Invention
The present invention relates to an optosemiconductor device having a superlattice configuration in which photo-excited carriers (excitons) are generated to change the transmission (or absorption) of light through the superlattice configuration. The optosemiconductor device can be used as an optical switch, optical bistable device, and optical memory in a high speed optical communication system and a high speed optical computer.
2. Description of the Related Art
The nonlinear properties of the transmission, absorption, and refraction of a superlattice configuration by excitons in multiple quantum wells (MQWs) are applied to high speed optical devices.
A prior art GaAs/AlGaAs MQW is formed by a superlattice configuration consisting of GaAs quantum wells and AlGaAs potential barriers in stacked films (see: A. Migus et al., "One-picosecond NOR gate at room temperature with a GaAs/AlGaAs multiple-quantum-well nonlinear Fabry-Perot etalon", Appl. Phys. Lett. 46 (1985) 70). Note, this GaAs/AlGaAs MQW includes only one kind of quantum well. In this MQW, if a light beam having a wavelength .lambda. corresponding to an electron energy level in the GaAs quantum wells is exposed to the MQW and satisfies a resonant tunneling condition: EQU m .lambda./2 =n L (1)
wherein
n is a refractive index of the MQW; PA1 L is a length of the MQW; and PA1 m is an integer,
a resonant tunneling phenomenon occurs, i.e., the absorption ratio of such a light beam by the MQW is remarkably reduced (i.e., the transmission ratio of such a light beam is remarkably increased), and this means that the light beam passes through the MQW.
When a next light beam having the same wavelength .lambda. is immediately made incident on the MQW, little of this light beam passes through the MQW, since the refractive index of the MQW is changed by the excitons in the quantum wells formed by the previous light beam and the Coulomb interactions between tunneled electrons. Nevertheless, when a sufficient time such as 30 ns has passed, the electron-hole pairs of the excitons are recombined, and thus another light beam can pass through the MQW.
Namely, the transmission ratio of a light beam of the MQW is initially quickly reduced but the transmission ratio of a light beam of the MQW is evenutally slowly increased, since a recovery time of the MQW is dependent mainly upon the recombination of electron-hole pairs of excitons formed in the quantum wells.
To reduce the recovery time, a new super-lattice configuration, called a "tunneling bi-quantum-well" (TBQ) configuration, has been suggested (see: Atsushi Tackeuchi et al.: "Fast Recovery of Excitonic Absorption Peaks in Tunneling Bi-Quantum-Well Structures", JJAP, vol. 28, No. 7, 1989, pp. L 1098-L1100). In this TBQ configuration, narrow and wide quantum wells are coupled by potential barriers, and the quantum energy levels of the narrow and wide quantum wells are different. For example, the height of a quantum energy level of the narrow quantum wells is higher than that of a quantum energy level of the wide quantum wells. When a light beam having a wavelength corresponding to a quantum energy level of the narrow quantum wells is made incident on the TBQ configuration, excitons are formed in the narrow quantum wells to reduce the transmission ratio of a light beam. In the TBQ configuration, the excitons formed in the narrow quantum wells are tunneled through the potential barriers to the wide quantum wells and thus the excitons formed in the narrow quantum wells disappear therefrom due to the tunneling of electrons and holes toward the wide quantum wells, in addition to the recombination of electron-hole pairs of the excitons. Accordingly, a recovery time of the transmission ratio can be shortened, for example, to on the order of 10 ps, to thereby enable a rapid switching of a light beam. The TQB configuration will be explained later in more detail.
In the above-mentioned TBQ configuration, however, the recovery time is still not satisfactory.